Brushless mechanico-electrical transducer



Oct. 9, 1962 A. M. COHEN 3,057,979

BRUSHLESS MECHANICo-ELECTRICAL TRANSDUCER 2 Sheets-Sheet 1 Filed June 7, 1960 INVENTOR.

f2 M2M J Oct. 9, 1962 A. M. COHEN 3,057,979

BRUSHLESS MECHANICO-ELECTRICAL TRANSDUCER Filed June 7, 1960 2 Sheets-Sheet 2 BYQMW/ United States Patent O M 3,057,979 BRUSH-BLESS MEtJHANlC-ELECTRICAL TRANSDUCER Arthur M. Cohen, Sturgis Highway, Westport, Conn. Filed June 7, 1960, Ser. No. 34,548 Claims. (Cl. Zilli-91) The present invention relates to a device for converting mechanical movement `into corresponding electrical variations.

There are many instances where mechanical movement or change in mechanical position are to be converted into corresponding electrical changes. One of the simplest forms of such device is the conventional rheostat or variable resistor, in which movement of a mechanical control element such as a shaft causes a corresponding change in the resistance of a resistor mechanically connected thereto. Many more sophisticated variations of this general theme are utilized in servo systems or telemetering systems. For example, sensed variations in the increasing diameter of `a takeup roll in `a thread winding machine can be used to 'control the rotational speed of the motor driving that roll `so as to achieve constant tension in the thread (servo system), or the physical condition of a Bourdon tube which senses pressure can give rise to an electrical change in a recording or transmitting device (telemetering system).

Many problems arise in the design and use of systems and devices of this character. Sensitivity is always an important factor. The power handling capacity of the device is another important factor, usually antithetical to sensitivity. If large amounts of power are to be handled directly by the transducer device itself, it must usually be so constructed as to be inherently relatively insensitive. Reliability and dependability are factors which are related to sensitivity and power handling capacity. In general, the more sensitive the device, and the more power it must directly handle, the shorter is its operating life and the more susceptible it is to improper operation.

Sensitivity is itself a fairly broad term which includes many characteristics within its scope. One aspect of sensitivity has to do with the amount of force which is required to be exerted on the mechanically moved element in order to give rise to a change in the controlled electrical characteristic. Another important aspect of sensitivity has to do with the essential nature of the controlled electrical characteristic and the ability of the device accurately to reflect very small changes in the mechanical position of the controlling element. lf the electrical characteristics can be varied only in discrete steps the sensitivity of the device is obviously quite limited.

The usual form of rheostat or variable resistor comprises a resistance wire helically wound about a supporting form, a brush being slidable along the form so as to make contact with different portions of the winding. With a device of this type uniform or continuous variation of the resistance is not feasible, since the movement of the brush from one operative position to its next adjacent operative position involves the connection to or disconnection from the circuit of a predetermined length of resistance wire having its own inherent resistance. Moreover, the overall electrical characteristics of devices of this type are dependent upon the electrical connection between the brush and the winding, and this is in turn dependent upon the mechanical 'and electrical condition of those two elements and the physical pressure with which the brush engages the winding. These factors are particularly sensitive to wear, to the deleterious effect of dust and dirt, to oxidizing and pitting of the brush, and to the amount of power which is directly handled by the device.

Another type of mechanico-electrical transducer com- 3,057,979 Patented Oct. 9, 1962 ICC prises 'a plurality of contact pairs adapted to be sequentially opened and closed in accordance with the movement of a control element. With a device of this type the disadvantages inherent in the use of a sliding brush are not present, but the electrical output of the device can be varied only in a distinctly stepwise manner, so that its sensitivity leaves much to be desired.

Neither the brush type rheostat nor the sequentially opened contact pair variable resistor has in the past been capable of general use in accurate servo systems, and their use in sensitive telemetering systems has been limited to those where only very small amounts of energy need be handled (signal energy as distinguished from power energy), largely because of the drawbacks set forth above. By way of contrast, the device of the present invention, by eliminating `the disadvantages of the prior art structures, is eminently capable of use both in highly sensitive telemetering installations having apreciable direct power handling characteristics and in delicate or critical servo system loops. lt converts linear or angular mechanical motion into continuous, dependable electrical variation with an accuracy and sensitivity which is comparable to that of much more complicated, expensive and less reliable electronic devices. It is capable of handling directly comparatively large amounts of energy, and hence permits tremendous power gains. The relationship between mechanical movement and the change in electrical characteristics can be varied widely, through the selection or variation of the external resistors or other impedance elements which the device controls, so that the device can be used in a Wide variety of applications. Indeed, since control of this relationship is accomplished by means of resistors which are `separate from the control unit itself, the identical control unit can be employed for many different installations, thus making for a high degree of standardization and flexibility of use. The device is exceptionally well insulated against corrosion, oil or dusty atmospheres and operates reliably in a wide range of temperatures. A single unit with its associated external resistors can dissipate up to 200 watts and can directly control circuits in the low kilowatt range. There is a complete absence of brushes and bearings. All of these factors combined give the device a life expectancy of up to 100,000 hours of continuous operation in moderate power applications.

These extremely significant results are achieved by a sophisticated mode of actuation of a contact-controlling member which causes the sequential opening and closing of a plurality of pairs of electrical contacts. An external mechanically moving element provides basic control for the position of that member. This alone, which has been done in the past, would produce an output variation which would be stepped in character, each step corresponding to the opening or closing of a given contact pair. In order to produce a stepless output, there is superimposed upon the mechanical positioning of said member by the external mechanically moving `element a dither action. This may be accomplished in a facile manner by energizing an electromagnet coil with a suitable alternating current such as the usual volt 60 cycle current which is almost universally available, that electromagnet being active upon a magnetizable element which is operatively connected to said contact-controlling member. The dither is sufficient to cause the member to vibrate so as to open and close a given contact pair while the external mechanically moving element remains in a given position. The length of time that a given contact pair will be opened or closed will depend upon the precise position of the member relative to that pair of contacts. If the member, as it approaches a given contact pair, is designed to open that pair of contacts, then the closer it is to them while it is being dithered the longer will that contact pair remain open during each cycle of the dither. A time integrating or averaging effect is produced which results in truly stepless or continuous variation of the controlled electrical characteristic.

The dither action is applied to the contact-controlling member in opposition to a biasing spring, the dithering force substantially balancing the biasing force of that spring, so that only exceptionally small forces need kbe applied to the external mechanically moving element in order to cause the operative position of the member to change, thereby to vary the electrical output. The external mechanically moving element is preferably connected to the contact-controlling member by a resilient part which decouples the dither from the external element. As is well known, the application of dither to an element which is to be moved greatly minimizes static friction, and this too adds to the sensitivity of the device even when exceptionally small control forces are applied thereto. Y The construction of the device is such that it can be housed within a sealed container, thus making it virtually insensitive to deleterious external influences. A device taking up a space no more than two inches in any direction can itself safely control up to 120 watts and be accurately, smoothly and continuously responsive to control forces as low as one gram.

To the accomplishment of the above, and to such other objects as may hereinafter appear, the present invention relates to a mechanico-electrical transducer as defined in the appended claims and as described in this specication, taken together with the accompanying drawings, in which:

FIG. l is a three-quarter perspective view of one embodiment of the present invention, shown encased and sealed;

iFIG. 2 is a schematic view of the operative parts of the device of FIG. 1;

FIG. 3 is a side elevational view of the internal moving parts of the device of FIG. 1, the base and housing being shown in cross section;

FIG. 4 is a three-quarter perspective view of a device such as shown in FIG. 1 .with a mechanical attachment designed to cause it to be controlled by rotary movement of an external element;

FIG. 5 is a cross sectional view taken along the line 5--5 of FIG. 4; and

FIG. 6 is a cross sectional View taken along the line 6 6 of FIG. 5.

Turning first to the schematic showing in FIG. 2, the device comprises a plurality of contact pairs generally designated Za-Zj and each comprising a xed contact 4 and a movable contact ringer 6 which is normally urged into engagement therewith. A plurality of resistors 8a- 81' are connected to and between the contact pairs Za-j and are electrically connected between terminals 10 and and 12. The number of resistors eiectively in the circuit between terminals 10 and 12. will depend upon the condition of the various contact pairs Za-j. Thus if all of the contacts 2a-j are closed leads 1.4 and 16, in conjunction with the closed contact pair 2a, will define a shunt around all of the resistors Sa-j, and none of those resistors will effectively be in the circuit. When con-tact pair 2a opens resistor 8a is inserted in the circuit. When contact pair 2b opens, which will occur after contact pair 2a has opened and while 2a remains open, resistors 8a and 8b will be in the circuit, and so on. Y

The contact pairs Za-j are sequentially opened by means of a finger-lifting (contact-controlling) member 18 having an inclined iinger-engaging edge 20 adapted to sequentially engage the movable contact fingers 6 as the finger-lifting member 18 moves downwardly, thereby to sequentially separate those movable contacts 6 from their respective xed contacts 4. Biasing springs 22 urge the linger-lifting member 18 upwardly away from the movable contacts 6. AT he nger-lifting member 18 is connected to an actuator arm 2,4 which is mounted to pivot freely about Vcurrent in the winding 36 alternates.

fulcrum 26. A resilient strip 23 extends from the actuating arm 24 and is provided with fingers 30 adapted to secure thereto an external mechanically moving element 32 which, in the form here shown, is adapted to move up and down, as indicated by the arrow 34. In order to provide the dither that portion of the actuating arm 24 extending to the right of the fulcrum 26 as viewed in FIG. 2 is formed of a magnetizable material such as soft iron. An electromagnet coil 36 has a core 38 which terminates close to the magnetic portion of the actuating arm 34 and the coil 36 is electrically connected to a source 40 of alternating current, such as the usual 11S-volt 60 cycle power supply.

When the coil 36 is energized the actuating arm at 24 will be subjected to a magnetic pull against the action of the springs 22 which will substantially balance the force of those springs. That magnetic pull will fluctuate as the This fluctuation Iwill cause the actuating armv 24, and with it the finger-lifting member 18, to dither. The frequency of this dither will be determined by the frequency of the current in the Winding 36. The amplitude of the dither, measured at the finger-lifting member 13, will ordinarily be somewhat less than the distance which that member must move in order to open or close an additional contact pair. VThe position of the actuating arm 24, and hence of the finger-lifting member 18, will be determined by theV position of the external mechanically moving element 32;. Y As that element moves upwardly as viewed in FIG. 2 the finger-lifting member 18 will be moved downwardly, opening more and more of the contact pairs 2 and thus placing more and more of the resistors Sa-j in the external electrical circuit to which the terminals 10 and l2 are connected. The external mechanically moving element 32 will, however, not rigidly determine the location ofthe finger-lifting member 18. Actually it will only determine a nominal position therefor, from which nominal position the iinger lifting member 18 will vibrate as it dithers.

In order to understand the mode of operation of the device, we may assume for purposes of explanation that a given nominal position of the linger-lifting member is such that has opened contact pair 2d and is halfway toward the position where it would open contact pair 2e. If there were no dither the effective resistance in the circuit between terminals ltl and l2 would be the sum of resistances Srl-tid, and this would be the case whether the iinger-lifting member. 18 were in the assumed nominal position or somewhat operatively closer to the contact pair 2e. rI'hus without dither there would be an appreciable degree of movement permitted to the finger-lifting member llS which would not result in any effective change in the characteristics of' the circuit between the terminals 10 and l2.

With dither, however, the situation is quite dierent. Let us assume that all of the dither movement is such as to move the finger-lifting member 1S toward the contact pair Ze and back to its nominal position. Under k these circumstances the dither will cause the contact pair 2e to be opened during half of each dither cycle and to be closed for the other half of that dither cycle. Therefore, integrated and averaged over a period of time, the effective resistance between terminals I0 and 12 will be the sum of resistors Srl-d plus one-half the value of resistor S3. If now the external mechanically moving element 32 moves up slightly, so that the nominal position of the ringer-lifting memberllS is moved down toward the contact points 2e but not so far as to open that contact pair (insofar as the nominal position alone is concerned), the result will be that the dither causes the contact pair 2e to be open for more than half of each dither cycle, say for three-quarters of that cycle. Hence the effective resistance between terminals lltl and l2, averaged over time, will increase by a value equal to onequarter the resistance of resistor 8e. Y

Y Thus by applying dither an essentially stepped variation in electrical output has been converted to a continuous stepless variation.

In addition, since the attraction between the electromagnet 36 and the actuating arm 24 substantially balances the biasing action of the springs 22, and since the `dither substantially eliminates static friction in the moving parts, very little force is required, for the external mechanical moving element 32 in order to vary the position of the finger lifting member 18. In some instances the amount of force required may be as little as a single ram.

g Because the actuating .ar-m 24 is connected to the external mechanically moving element 32 by means of the resilient strip 28, the said external mechanically moving element 32 is effectively decoupled from, and unaffected by, the dither.

Referring now to FIG. 3, which discloses a specific mechanical construction in the form of a sealed plug-in standard unit embodying the features of the present invention, that unit comprises a base plate 42 above which a mounting plate 44 is supported by posts 46. The actuating arm 24 is mounted on the underside of the mounting plate 44 by means of leaf spring 48 and pivots about the left hand edge of the mounting plate 44 as a fulcrum. The finger-lifting member 18 is pivotally connected to the actuating arm 24 by means of an extension 50 of the leaf spring 4S. The fixed contacts 4 and movable contact fingers 6 of the several contact pairs 2a-j are mounted in .any appropriate manner on the base plate 42 with the tips of the movable contact fingers 6 extending out to the left beyond the fixed contacts 4 and into the path of movement of the lower edge 20 of the fingerlifting member 18. The various contacts are connected by appropriate leads to prong-type terminals 52.

Mounted on the support plate 44, and between it and a top plate 54 supported by posts 5S, is the electromagnet coil 36, its core 38 extending below the mounting plate 44 and being adapted to be received in aperture 56 in the actuating arm 24 when the latter is pivoted in a counter-clockwise direction as viewed in FIG. 3. Arms 58 extend to the left from the top plate 54, and the springs 22 are connected between the ends of the arms 58 and the linger-lifting member 18. A central arm 60 extends to the left from the top plate 54, and it carries a dashpot cylinder 62 within which piston 64 is slidable, that piston being connected by resilient arm 66 to the finger-lifting member 18. A shield can 68 surrounds the operative parts of the mechanism and is sealed to the base plate 42, as by being received inside flanges 70 formed thereon. An aperture 72 is formed in one side of the shield can 69 through which the resilient connecting strip 2S extends. A flexible rubber diaphragm 74 seals the aperture 72 and is sealingly connected to the resilient strip 28, thereby permitting the latter to move in .accordance with the movement of the external mechanically moving element 32, while at the same time preventing the entry of any deleterious elements into the shield can 68.

The base plate 42 may be provided with a mounting iiange 76 provided with apertures 78 so that it may be secured to any appropriate panel mounting.

In the embodiment of FIGS. 1 3 the external mechanically moving element 32 is adapted for linear vertical movement in the direction of the arrow 34. The embodiment disclosed in FIGS. 4-6 involves a special attachment for permitting accurate control of the device by means of a rotatable external mechanically moving element 32. That attachment comprises a support housing having a top wall 80 and side walls 82, which latter have flanges 84 adapted to fit over the anges 76 of the unit of FIG. l and having apertures 86 adapted to register with the apertures 78 in the flanges 76, thereby to secure the attachment to the basic unit. The external mechanically moving element 32', which takes the form of a control shaft to which ya knob 83 is secured, extends 6 through a suitable bushing 88 mounted in the top wall Sil, the shaft 32 carrying a cam 90 at its lower end and being held in axial position within the bushing 8S by means of lock ring a2. The outer surface of the cam 90 is shaped to provide a predetermined relation between rotation of the shaft 32 and movement of the resilient strip 23 of the control unit, and also carries an upstanding stud 93 which cooperates with appropriately positioned stops 94 which depend from the top wall 30. A pair of posts 96 extend inwardly from an end wall 9S of the housing, and a leaf spring 100 is secured to and extends between those posts. The central portion of the leaf spring 100 is provided with a pair of apertures 102 through which the tips 104 of framework rods 106 extend, an L-shaped bracket 108 being secured to the rods 1% adjacent their tips 104 in any appropriate manner, as by welding. The horizontal portion of the bracket 108 engages the leaf spring 10() and the upstanding portion thereof engages the outer surface of the cam and constitutes a cam follower. The framework rods 106 extend to the right, as viewed in the figures, into a position over the aperture 72 in the basic unit, and there the rods 106 are connected by a cross piece 110 secured in place in any appropriate manner, as by welding, the lingers 30 of the resilient strip 23 which extends out through the 4aperture. '72 in the shield can 68 of the basic unit engaging that cross piece 110. The leaf spring 100 urges the cam follower 103 against the cam 90 and, by reason of the shaping of the external surface of the cam 90, rotation of the shaft 32 is translated into longitudinal movement of the cross bar 101i, thus correspondingly moving the resilient strip 28 and hence the fingerlifting member 18.

Thus the basic control device of the present invention, as disclosed in FIGS. 1-3, constitutes a standard unitary structure capable of transforming mechanical movement or change of position into corresponding electrical variation in an accurate, continuous, smooth and stepless manner. The actual relationship between the mechanical movement of the external element 32 and the variation in electrical characteristics is determined by the choice and/or variation of the externally mounted resistors Saz-j, thus producing a high degree of flexibility and adaptability. Any desired taper can be achieved, and if adjustable resistors are employed a given unit can be used for many different purposes or for laboratory purposes.

The embodiment here specifically disclosed, which utilizes ten contact pairs, can control up to watts. If twenty contact pairs are employed the power handling capacity is approximately doubled. These power handling capacities permit the units to be used for direct control in low kilowatt ranges. Rugged, dust-tight, tamper-proof construction and the complete absence of brushes and bearings ensure a life expectancy up to 100,000 hours in moderate powerapplications. A total mechanical movement of the external mechanically moving element 32 of approximately la" Will provide control over the entire range of operation of the device, and the amount of force involved to achieve control may be as low as l gram.

While the device has been here .disclosed as controlling an external circuit comprising resistors, other impedance elements could be used in place `of resistors. The impedance elements may be connected in series, as shown, or in parallel, for various types of control characteristics. The member 18 has been termed a finger-lifting member because in the embodiment here specifically disclosed its downward movement sequentially lifts the movable contacts fingers 6 from their respective fixed contacts 4, but -by a simple mechanical modification the same movement of the member 1S could be used to permit the movable contact fingers 6 to engage their respective fixed contacts 4 to close the contact pairs. The use of contact fingers 6 is not essential to the instant invention, and the so-called finger-lifting member'l'couldactuate coritact pairs between open and closed positions in other ways. Hence the terms finger and lifting should not be given a restrictive significance. Many other variations may be made in the specific structures here disclosed, all without departing from the spirit of the invention, as delined in the following claims.

I claim:

l. A mechanico-electrical transducer comprising a iixed contact, a movable contact normally urged into engagement with said fixed contact and deining therewith a contact pair, said contacts being adapted to be electrically connected to electrical circuitry so as to vary the electrical character thereof in accordance with the engagement or disengagement of said contact pair, a. contact-controlling member articulately mounted relative to said movable contact and operatively connected to said movable contact so as to control its position relative to said tixed contact, means operatively connecting said contact-controlling member to an external mechanically moving element, and an A.C.energized coil operatively connected to said contact-controlling member to impart vibratory movement thereto in response to the A1C. variation in energization of said coil7 thereby to cause translducing to be eiiective in a substantially stepless manner.

2. In the transducer of claim l, a housing having an aperture, said means connecting said contact-controlling member to said externally mechanically moving element passing through said aperture and being in part external of said housing, the other named parts of said transducer being contained within said housing, and a flexible sealing element extending between and engaging said housing and said connecting means and sealing said aperture.

3. The transducer of claim l, in which said means operatively connecting said contact-controlling member to an external mechanically moving element comprises a resilient part located between said external mechanically moving element and the point at which said coil is oper- -atively connected to said finger lifting member.

4. In the transducer of claim 3, a housing having an aperture, said resilient part passing through said aperture and being in part external of said housing, the other named parts of said transducer being contained within said housing, and a flexible sealing element extending between and engaging said housing and said resilient part and sealing said aperture.

5. A mechanico-electrical transducer comprising a plurality of xed contacts, a plurality of movable contacts normally urged into engagement with said xed contacts and defining therewith a plurality of contact pairs, said contacts being adapted to be electrically connected to electrical circuitry so as to vary the electrical character thereof in accordance with the number of contact pairs in engagement with one another, a contact-controlling member articulately mounted relative to said movable contacts and operatively connected to said movable contacts so as to sequentially separate them from their respective xed contacts, means operatively connecting said contact-controlling member to an external mechanically moving element, biasing means operatively connected to said contact-controlling member for urging it to move in a give ,direction relative to said moving contacts, and an A.C.energized coil operatively connected to said contact-controlling member to urge it in a direction opposite to said given direction and substantially balancing the eiiect of said biasing means and to impart vibratory movement thereto in response to the A.C. variation in energization of said coil, thereby to cause transducing to be effective in a substantially stepless manner.

6. In the transducerV of claim 5, a housing having an aperture, said means connecting said contact-controlling member to said externally mechanically moving element passing through said aperture and being in part external of said housing, the other named parts of said transducer being contained within said housing, and a iiexible sealing element extending between and engaging said'housing and said connecting means and sealing said aperture.

7. The transducer of^claim 5, in which said means operatively connecting said contact-controlling member to an external mechanically moving element comprises a resilient part located between said external mechanically moving element and the point at which said coil is operatively connected to said finger lifting member.

8. In the tranducer .of claim 7, a housing having an aperture, said resilient part passing through said aperture and being in part external of said housing, the other named parts of said transducer being contained within said housing, and a iiexible sealing element extending between and engaging said housing and said resilient part and sealing said aperture.

9. A mechanico-electrical transducer comprising a plurality of xed contacts, a plurality of movable contacts normally urged into engagement with said fixed contacts and defining therewith a plurality of contact pairs, said contacts being adapted to be electrically connected to electrical circuitry so as to vary the electrical character thereof in accordance with the number of contact pairs in engagement with one another, a contact-controlling member articulately mounted relative to and operatively connected to said movable contacts and movable toward and away from said movable contacts so as to sequentially separate them from their respective lixed contacts, an actuator pivotally connected to said contact-controlling member, extending substantially at right angles thereto, and itself mounted for pivotal movement about a fulcrum, said actuator having a magnetic portion, a part extending from said actuator and adapted to be operativelyV connected to an external mechanically moving element, andV @an A.C. energized 4electromagnet oper- :atively connected to said magnetic portion of said actuator to impart vibratory movement thereto, and through it to said contact-controlling member, in response to the A1C. variation in energization of said electromagnet, thereby to cause transducing to be effected in a substantially stepless manner.

l0. In the transducer of claim 9, a housing having an aperture, said part extending from said actuator passing through said aperture and being in part external of said housing, the other named par-ts of said transducer being contained within said housing, and a iiexible sealing element extending lbetween and engaging said housing and said part and sealing said aperture.

11. The transducer of claim 9, in which said part extending from said actuator is resilient.

l2. In the transducer of claim 1l, a housing having an aperture, said resilient part passing through said aperture and being in part external of said housing, the other named parts of said transducer being contained withinV said housing, and a flexible sealing element extending between'and engaging said housing and said resilient part and sealing said aperture.

13. A mechanico-electrical transducer comprising a plurality of fixed contacts, a plurality of movable contacts normally urged into engagement with said fixed contacts and defining therewith -a plurality of contact pairs, said contacts being adapted to be electrically connected to electrical circuitry so as to vary the electrical character thereof in accordance with the number of contact pairs in engagement with one another, a contactcontrolling member articulately mounted relative to said movable contacts and movable toward and away from said movable contacts so as to sequentially separate them from their respective fixed contacts, biasing means operatively connected to said contact-controlling member for urging it to move in a given direction `relative to said movable contacts, an actuator pivotally connected to said contact-controlling member, extending substantially at right angles thereto, and itself mounted `for pivotal movement about a fulcrum, Ysaid actuator having -a magnetic portion, a part extending from said actuator and adapted to be operatively connected to an external mechanically moving element, and an A.C.energized electromagnet operatively connected to said magnetic portion of said actuator to urge it in a direction opposite to that corresponding to said given direction of said contact-controlling member and substantially balancing the effect of said biasing means and to impart vibratory movement thereto, and through it to said contact-controlling member, -in response to the A.C. variation in energization of said electromagnet, thereby to cause transducing to be effected in a substantially stepless manner.

14. In the transducer of claim 13, a housing having an aperture, said part extending from said actuator passing through said aperture and being in part external of said housing, the other named parts of said transducer being contained within said housing, and a flexible sealing element extending between and engaging said housing and said part and sealing said aperture.

15. The transducer of claim 13, in which said part extending from said actuator is resilient.

16. In the transducer of claim l5, a housing having an aperture, said resilient part passing through said aperture and -being in part external of said housing, the other named parts of said transducer being contained within said housing, and a flexible sealing element extending between and engaging said housing and said resilient part and sealing said aperture.

17. In combination with the transducer of claim 1, an external mechanically moving system comprising a housing, a control member rotatively mounted on said housing, a cam operatively connected to said control member for movement therewith, a cam follower engaging said cam, a spring mounted on said housing and engaging said cam follower and resiliently urging it against said cam7 and a member operatively connected to said cam follower and carrying said external mechanically moving element to 'which said contact-controlling member is operatively connected.

18. In combination with the transducer of claim 2, an external mechanically moving system comprising a housing, a control member rotatively mounted on said housing, a cam operatively connected to said control member for movement therewith, a cam follower engaging said cam, a spring mounted on said housing and engaging said cam follower and resiliently urging it aginst said cam, and a member operatively connected to said cam follower and carrying said external mechanically moving element to which said contact-controlling member is operatively connected, said external mechanically moving system comprising a unitary structure receivable as such 10 on said transducer housing over said aperature in said housing.

19. In combination with the transducer of claim 1, an externally mechanically moving system comprising a housing, a control shaft passing through and rotatively mounted on said housing and being manually accessible at the exterior of said housing, and, inside said housing, a cam operatively connected to said control member for rotation therewith, a cam Ifollower engaging said cam on the side thereof remote from said finger lifting member operative connection means, a spring mounted on said housing and engaging said cam follower so as to urge it against said cam, and a pair of members operatively connected to said cam follower and extending therefrom to either side of said control shaft toward said contact-controlling member operative connection means, said framework elements carrying said external mechanically moving element to which said contactcontrolling member is operatively connected.

20. In combination Awith the transducer of claim 2, an externally mechanically moving system comprising a housing, a control shaft passing through and rotatively mounted on said housing and being manually accessible at the exterior of said housing, and, inside said housing, a cam operatively connected to said control member for rotation therewith, a cam follower engaging said cam on the side thereof remote from said finger lifting member operative connection means, a spring mounted on said housing and engaging said cam follower so as to urge it against said cam, and a pair of members operatively connected to said cam follower and extending therefrom to either side of said control shaft toward said contactcontrolling member operative connection means, said framework elements carrying said external mechanically moving element to which said contact-controlling member is operatively connected, said external mechanical moving system comprising a unitary structure receivable as such on said transducer housing over said aperture in said housing.

References Cited in the file of this patent UNITED STATES PATENTS 1,134,058 Parris Mar. 30, 1915 2,076,115 Benit Apr. 6, 1937 2,550,779 Cohen May l, 1951 2,580,732 Cohen Ian. 1, 1952 2,650,967 Cohen Sept. 1, 1953 2,707,735 Cohen May 3, 1955 2,875,383 Cohen Feb. 24, 1959 

